Pipelines for transporting hydrocarbons are composed of pipe sections joined together to cover overall lengths in the order of hundreds of kilometres. The pipe sections have a unit length, generally 12 meters, and relatively large diameters of between 0.2 and 1.5 metres. Each pipe section comprises a steel cylinder, a coating in a polymer material that has the function of protecting the steel pipe, and a possible further coating in Gunite or concrete, the function of which is to weigh down the pipeline in underwater applications.
Typically, pipelines are buried close to shore approaches and in relatively shallow waters to protect them from damage caused by blunt instruments such as, for example, anchors and fishing nets. Sometimes, pipelines are buried to ensure protection against natural agents such as wave motion and the current, which could overstress the pipelines. As such, when the pipeline is laid on the bed of the body of water it can happen that the pipeline is placed with a span (i.e., the pipeline has a raised section with respect to the bed located between two support areas). In the described configuration, the pipeline is particularly exposed to wave motion and the current, and offers relatively little resistance to displacements induced by these natural agents. In other cases, the need for burying is due to thermal instability problems that cause deformation of the pipeline (upheaval/lateral buckling). In yet other cases, it is necessary to protect the pipeline from the mechanical action of ice that, in particularly shallow waters, can scour the bed.
To bury a pipeline in a bed of a body of water, it is necessary to dig a trench, place the pipeline on the bottom surface of the trench and bury the pipeline. Different techniques have been proposed for this purpose.
In general, the laying of a pipeline in a body of water is carried out by a floating unit, which, while the floating unit proceeds along a predetermined route, releases the pipeline in the body of water by a lay device. The pipeline has a spanned section between the lay device and the bottom of the trench or the bed of the body of water.
There are substantially two burying methods: the pre-trenching method, where a trench is dug and the pipeline is laid directly on the bottom surface of the trench, and the post-trenching method, in which the pipeline is laid on the bed of the body of water and the trench is then excavated directly beneath the pipeline.
There are many trench excavation methods, including: dragging a plough that forms a furrow in the bed of the body of water; advancing a milling cutter to break up a strip of the bed of the body of water and devices configured to remove the strip of broken-up bed, such as Archimedean screws for example; and advancing a milling cutter to break up a strip of the bed of the body of water and fluidization devices to make the pipeline sink into the broken-up and “fluidized” ground mass.
It is also known to support the lateral surfaces of trenches with movable lateral support walls when the trenches are characterized by very high depth/width ratios, to prevent the trench caving in before having correctly positioned the pipeline in the trench.
Furthermore, when the span between the lay device and the bottom of the trench is relatively very long, the lateral walls must also be relatively particularly long.
The following documents reveal the above-indicated known techniques: PCT Patent Published Application No. WO 2009/141,409 A2, European Patent No. EP 2,501,864 B1, U.S. Pat. No. 4,710,059, U.S. Patent Application Publication No. 2012/0057940 A1, European Patent No. EP 2,331,754, U.S. Pat. Nos. 3,822,558, and 4,588,329.
It is evident that the difficulties associated with the burying of the pipeline as well as the energy necessary for the burying increase with the burying depth and with the span length of the pipeline. Further critical problems arise when laying is carried out in relatively difficult environments, such as the Artic Sea for example, where the scouring caused by icebergs is quite considerable and imposes burying the pipeline relatively very deeply. In addition, the relatively short season available for laying the pipeline dictates carrying out the laying relatively very quickly. It is also evident that the severe restrictions imposed by the environmental conditions must not be detrimental to the integrity of the pipeline.